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On Mars, twice as far from the Sun as Earth is, an evacuated tube solar thermal collector will still reach temperatures of 300C to 500C because there's no convective cooling to speak of. This is sufficient to store large amounts of sensible heat energy in liquid Lithium metal for powering motor vehicles in a practical way. Although pure Lithium metal at 500C will aggressively attack most metals and ceramics, a handful of alloys, such as Molybdenum-Rhenium alloys, do not exhibit ductile-to-brittle transformations from being repeatedly heated and cooled in thermal power cycles. Impurities in the Lithium, such as Oxygen or Nitrogen, must be removed, irrespective of which alloys are used to store the molten Lithium, but certain alloys of Tantalum, Molybdenum, Rhenium, Tungsten, and Zirconium are all suitable for storing molten Lithium at elevated temperatures, even with trace contaminants included. 500C is within the safe working range for Molybdenum-Rhenium and various Tungsten alloys, although Tungsten alloys may also require Zirconium to not suffer from embrittlement. Either way, highly pressurized Argon would either inject heat into or extract heat from a tank of molten Lithium metal via Molybdenum-Rhenium alloy tubing within the storage tank.
If we can heat the Lithium to 500C temperatures during daylight hours and the ambient environment is 0C, then the heat storage capacity of pure molten Lithium metal is about 670Wh/kg. Average surface temps tend to be below 0C, but we'll presume 0C for sake of argument because max summer averages near the equator are only 20C or so. This is at least as good as any conventional Lithium-ion battery at the pack level, even after accounting for efficiency losses associated with converting thermal power into electricity. A Lithium metal thermal battery doesn't appreciably degrade from repeated charge-discharge cycles, nor does it require any sensitive microelectronics to control power input and extraction. The solar and GCR radiation environment has no appreciable effect on these materials at the time scales involved with typical motor vehicle usage, either. A vehicle so-powered could continue to be charged and recharged for many years without appreciable loss of energy storage capacity.
After accounting for inefficiencies, a land motor vehicle should still be able to extract 1/3rd of the energy from the molten Lithium, meaning 200Wh/kg. The best production car EV batteries used here on Earth are 150-250Wh/kg. Solid state batteries may do better, but we have no idea how well these will hold up, long-term, nor what temperature limits they'll be subjected to.
It would still be prudent to crash-protect a molten Lithium battery because hot Lithium metal is so reactive with most metals and ceramics, but the supercritical Argon is non-reactive, so a pinhole leak in the thermal power transfer loop might rupture the battery casing but it won't explosively react with the Lithium. I suppose liquid Sodium metal could transfer thermal power so there is no pressurized fluid inside the battery itself. Sodium is still very reactive, but nowhere near as bad as Lithium at 500C, and this is Mars, so very little O2 or water in the atmosphere.
This will be a very expensive and physically large battery due to Lithium metal's very low density, but as long as you can heat it to operating temperature, whether directly or indirectly using electricity, it's thermal energy storage capacity never changes. The same cannot be said for any kind of electro-chemical battery, which always degrade from charge/discharge cycling and thermal excursions associated with excessive charge/discharge rate.
A vehicle could not rapidly recharge itself on the move, no different than here on Earth, but a thermal power collector mounted to the top of the vehicle would be sufficient to cover daily life support power requirements for its occupants. Solar thermal power collectors would not be subject to permanent damage from the abrasive blowing dust in the Martian atmosphere, nor any appreciable degradation over time from UV damage, as would be the case with photovoltaics. We'd still want to convert the output into electricity for ease of use in powering the vehicle and onboard electronics / lights / life support systems. However, the energy generation and storage system is mechanical and uses very few moving parts. When the solar input power stops flowing, a thermal energy generation and storage system has no "light switch" effect, unlike photovoltaics and electro-chemical batteries. This is an important occupant and system protection feature. The power system cannot "move" at the same speed as electricity, but that is generally a good thing when reliability is more important than achieving an unrealistic efficiency target.
More recent experiments using CNT fabrics to efficiently transfer heat into heat pipe systems using comparatively large fabric surface areas may allow much larger thermal power collection areas than could realistically be mounted to a vehicle, but this would require substantial testing prior to relying on it to deposit more thermal power into the vehicle's battery. The most interesting application of this tech is providing functionally unlimited power for EVA. Water losses, consumables depletion, and the surface radiation environment would still limit EVA time, but when your suit can constantly recharge itself or power life support equipment using ambient temperature deltas, that's a very high value-add to the mission.
For those wondering how that might work, certain Gallium eutectic metal mixtures convert CO2 into pure Carbon powder and O2 at room temperature without any thermal or electrical power input. The thermal power transfer system that the astronaut is wearing consumes waste body heat and absorbs input thermal power from the Sun during daylight hours, which it converts into pumping power, using a radiator, to maintain O2 levels and suit pressurization. The suit wearer only needs thermal power for the pumps and blower motor, which can be powered by fluids like water. So long as body heat and the Sun create a sufficient temperature delta, the pumps keep spinning. Bottled liquid CO2 would be carried in the astronaut's life support system and converted into O2 on-demand, mostly as a supplemental O2 supply. Most of the time the Gallium eutectic mixture is stripping the Carbon from the wearer's exhaled CO2, and recycling it. O2 is about 72.71% of CO2 by weight, tanks that hold highly pressurized gaseous Oxygen are much heavier than liquid CO2 equivalents.
Pure O2 density at 66bar of pressure (max pressure for the PLSS tanks in the current space suit design) is only 0.09242kg/L. Liquid CO2 density at room temperature is about 0.74kg/L, so it contains about 0.53kg of pure O2, and pressure is only 58bar. That's 5.7X more extractable O2 carried at a modestly lower pressure. It's heavier because you're carrying the Carbon with you as well, but the size of the tank required for supplying an equivalent amount of O2 is greatly reduced by using Gallium eutectic to strip the Carbon from the CO2 and waste heat pumping power generation with CNT fabrics as thermal power collectors.
This clearly doesn't help with water or food, and your body obviously requires pressurization or mechanical counter-pressure, but your odds of suffocating immediately are reduced because the entire Martian atmosphere becomes your backup O2 supply. If you have the tech to collect and bottle it as LCO2, then the Gallium eutectic and CNT thermal pump take care of stripping Carbon. Maintenance is also required because Carbon will eventually accumulate atop the Gallium and prevent further CO2 from being bubbled through the metal. You need body heat or some kind of heat to prevent the Gallium from solidifying as well. Basically, the eutectic mixture needs to be kept at room temperature or higher, one way or another.
No electronic control system is required to make that process work, but it would likely benefit from more precise metering and monitoring that only electronics can provide. You still need some electrical power for lights, radios, life support system sensors, etc, but the amount of power consumed by the life support system is mostly limited to monitoring functions and periodic intervention to supply more O2 from the CO2 supply.
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This post is reserved for an index to posts that may be contributed by NewMars members.
While the topic provides focus upon Supercritical Argon, the opening post contains discussion of multiple technologies.
For those wondering how that might work, certain Gallium eutectic metal mixtures convert CO2 into pure Carbon powder and O2 at room temperature without any thermal or electrical power input.
Since every human who exits a habitat to venture out onto the surface of Mars will require life support, the line quoted above appears to me to have potential application for every citizen. That part of the opening post for this topic might eventually become a topic on it's own.
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